1. Field of the Invention
The present invention relates to an image projection system for illuminating an image formed on a light valve with light, and projecting the image on a screen by a projection lens.
2. Description of the Prior Art
To obtain a large picture, light is emitted from a light source to a light valve of a small size to form an optical image depending on the picture signal, and an enlargement of the optical image is projected on the screen by a projection lens. A projection system using a liquid crystal panel as the light valve is noticed. The conventional projection system (for example, as disclosed in the Institute of Television Engineers of Japan Technical Report, CE '89-17, pp. 55-60) comprised a lamp, a light collecting optical system for collecting the radiation light from the lamp, a color separation optical system composed of three dichroic mirrors, three field lenses, three active matrix liquid crystal panels, three projection lenses, and a screen. Because of the use of three projection lenses, the color combination optical means is not needed, and the projection lenses may be easily widened in angle. Therefore, a compact projection system is composed.
The light from the light source is a natural light, but the liquid crystal panel utilizes a linearly polarized light obtained through a polarizer. When the angle formed by the liquid crystal molecular alignment direction of the liquid crystal cell wall surface of the liquid crystal panel and the p-polarization direction of the dichroic mirror is about 45 degrees, the color light of the mean component of the p-polarization component and s-polarization component from the dichroic mirror enters the liquid crystal panel. Assuming the difference between the wavelength of 10% transmittance of the spectral transmittance characteristic of the dichroic mirror and the wavelength of 90% transmittance to be a gradient width, the gradient width of the mean component is greater than the gradient width of the spectral transmittance characteristic of the p-polarized and s-polarized light components. The greater the gradient width, the larger become the brightness and chromaticity changes with respect to changes of the spectral transmittance of the dichroic mirror due to incident angle dependence or film thickness unevenness. Besides, the color light of the mean component of the p-polarized and s-polarized light is larger in the change of the spectral transmittance characteristic of the dichroic mirror with respect to the incident angle, than the color light of the p-polarized or s-polarized light, and the color nonuniformity of the projected image becomes larger. If there is a slight fluctuation in the spectral transmittance characteristic of the dichroic mirror, the brightness and chromaticity of the projected image vary.